The present invention relates to a mechanism for transmitting power from an engine shaft to a power take-off (PTO) shaft.
A PTO transmission system with a planetary gear split power transmission unit is described in U.S. Pat. No. 6,692,395, issued on 17 Feb. 2004 and assigned to the assignee of this application. With this system, power can be transmitted from an engine to a PTO shaft, and the PTO shaft speed can be maintained constant regardless of variations in engine speed.
This transmission has an output connected to a PTO shaft, a first input connected to an engine output shaft and a second input connected to an output of a fixed displacement hydraulic motor. The motor is driven by a variable displacement hydrostatic pump which is driven by the engine output shaft. A control unit controls the speed of the variable speed drive unit as a function of sensed engine speed and to maintain the PTO shaft at a constant speed.
This system transmits power to the PTO shaft from both the engine and from the variable speed drive unit. Maintaining a constant PTO shaft speed allows PTO driven implements, such as a grass/hay spreader, to be run optimally. This system also increases fuel economy and reduces noise.
However, with this transmission, when the engine is running at speeds at or higher than rated engine speed, the sun gear on the planetary must be locked up so that all power is transmitted mechanically. This lockup of the sun gear can be accomplished by commanding the pump to zero displacement, which prevents rotation of the hydraulic motor that is coupled to the sun gear. However, this requires a larger motor with a large torque capacity in order to prevent rotation of the sun gear in this situation.